My question is why do some small engines use resistor spark plugs with a very small gap? I fought a chain saw that would start and run well but then would not start again after running a short time until the plug used was cleaned. After replacing with the proper resistor plug no more starting issues.
My clip on distributor must have had a resistor built into the rotor that lost continuity. The replacement Rotor did not have that resistor.
The clip on dizzy and chain saw are both German made.
The usual reason that resistor plugs and resistor spark plug wires are used is to greatly reduce the Radio Frequency Interference that the ignition system will cause to radio and TV reception that is nearby. Not sure what the regulations are in Germany but they may have stricter rules there. Using them on a T with wood box coils cam be fatal to the coils since the plug arc and solid wire plug wires will greatly limit the coil from producing too high of a spark voltage. The resistors used are usually rather high value so the voltage at the coil is not limited to anything safe for a wood box coil.
I don't have an answer to your question, but you may have just answered the long standing question of why my chainsaw has always done exactly what you describe.
I think one cause of the chain saw not starting is our new gas. The tank should be drained and the saw run until out of gas to fix the problem, then use fresh gas the next time you use the saw. I couldn't get mine to start and so I got an electric chain saw. This works well in the yard with a long extension cord, but would not be useful if you go out into the woods to gather wood.
As to the Model T, I found that the ignition system fouls AM radios for about 100'. I also agree that any resistance to the spark could damage the coil. The spark will find the easiest path to ground and there are wood parts in the coils and the coil box through which the spark can arc and burn a carbon path.
Had a Honda Trail 90 that only liked to run with new fresh plugs, sold it before I really got into it.
The rotor that went south on my clip on obviously had a more expensive design with wiring down into some sort of epoxy then back up to the outer end. The NAPA replacement has solid metal from the center to the end.
After checking resistance plugs I can now recognize them quickly by there cap. From now on though the right plug will be used as I often buy used machines and previous owners change plugs and gaps.
5mm is a very small gap. Thanks John!
My question is what kind of resistor are you talking about? Spark plugs come with two different types: pile/air gap, and inductive resistors. The first type are supposed to cut down on RFI as John and others have already said. The second type are more common on some two stroke engines with CD systems. I have never seen an inductive resistor plug application for a chainsaw. Most saws I have ever used or worked on had CD ignition systems, but never used resistor plugs. Using standard resistor spark plugs may adversely affect CD ign. systems and actually lower plug voltage. Inductive resistor plugs work very well with CD systems in the correct application. You didn't say what brand of chainsaw you're using, but I would double check the application literature and charts. My guess is they don't use resistor plugs, but the gaps on their standard plugs are pretty small (ie., .017-.018).
I don't know the type of resistor used on my 635 Solo chain saw. The information found on the web shows use Bosch WJR6F or Champion RCJ6Y plugs with a recommended gap of 5mm in the factory book. Ran it for an hour this morning----ran well with slight need of high low carb adjustment not done yet.